The present invention relates to a valve arrangement, especially for controlling flow of hot water through a heating device for motor vehicles. The arrangement mainly comprises a substantially closed housing having an inlet chamber and an outlet chamber communicating with each other through an opening forming a valve seat, which may be closed and opened by a valve member operated by an electromagnet within the housing. The arrangement includes further a flexible membrane clamped at its outer periphery to the housing and forming on one side of the inlet chamber facing away from the valve seat a control chamber. The electromagnet acts also on an auxiliary valve member arranged in the control chamber and cooperating with an auxiliary valve seat in the membrane in such a manner that during movement of the valve member in valve closing direction a higher pressure will be created in the control chamber than prevails in the outlet chamber and during movement of the valve member away from the valve seat a lower pressure will be created in the control chamber than in the inlet chamber.
A known hot water controlling valve of this kind, disclosed in the German Offenlegungsschrift No. 25 31 015 operates with underpressure as servo-force. For this purpose the auxiliary valve member comprises a leaf spring which is operated by the electromagnet. The leaf spring is arranged in the control chamber. It closes with its resilient end, when the electromagnet is deenergized and the opening between the inlet chamber and the outlet chamber is closed, an underpressure conduit communicating with the control chamber. If the opening through which the inlet chamber communicates with the outlet chamber has to be opened, the electromagnet has to be energized. It attracts then the leaf spring which in turn opens the underpressure conduit communicating with the control chamber and at the same time closes another opening through which the control chamber communicates with the surrounding atmosphere. In this way an underpressure will be created in the control chamber during opening of the valve. Thereby the valve member moves away against the action of a closer spring from the aforementioned opening so that the valve is opened and hot water may pass the valve and flow into a heat exchanger. In the valve closing position the electromagnet controlling movement of the valve member is deenergized, but in the valve open position the electromagnet has to be continuously energized, which is of disadvantage. A further disadvantage is that the opening movement of the valve depends on the magnitude of the underpressure created in the control chamber. This magnitude of the underpressure may, however, during certain operating conditions of the motor vehicle not be sufficient to always assure movement of the valve in opening direction. Thus, for instance, in motor vehicles constructed to prevent obnoxious exhaust gases to penetrate into the atmosphere, the underpressure in certain extreme full load conditions of the vehicle, is insufficient to assure an opening movement of the valve. This disadvantage occurs also in Diesel engine operated vehicles. A further disadvantage resides in that the valve will not open when the underpressure conduit communicating with the control chamber leaks or is inadvertently disconnected. The safety of operation of this known arrangement is also endangered in that during a leak of the membrane water may flow to the outside of the valve, which may lead to a malfunction of the heating arrangement and which may also lead to a damage of the whole heating arrangement and to the engine of the vehicle.